Electrochromic (EC) devices have been attracting widely-spread attention as they can be used as smart windows and electronic displays. In particular, recent research and development progress in organic and polymer electrochromic materials exhibiting different voltage-dependent colors makes EC devices a strong candidate for sunlight-readable exterior displays. Typically, an EC device includes an electrochromic material between two electrodes and in contact with an electrolyte. A porous layer, referred to as the docking layer, is prepared from a suitable semiconductor material such as TiO2 or ZnO, attached to one of the electrodes and separated from the other electrode by the electrolyte. The electrochromic material is absorbed or attached to the docking layer. When a high enough voltage is applied, the electrochromic material is reduced or oxidized, and changes color. For example, diethyl viologen diiodine is an electrochromic material which is colorless, and becomes darkly colored upon reduction.
However, the quest for electrochromic display technology often suffers from the dilemma of the thickness of the docking layer and the resulting slow charge diffusion that limits the switching speed of electrochromic device. Explicitly, a film with a large surface area such as a TiO2 nanoparticulate film or a polymer film is often desired to load enough electrochromic materials for sufficient color contrast, but at a cost of high driving voltage and slow response time due to the large series resistance and slow electron mobility in the docking layer. Once an electric leak occurs between the two electrodes, the high voltage will immediately drop on the electrolyte, resulting in dielectric breakdown of the electrolytes and active electrochromic material, thus deteriorating the lifetime of the device.